(a) Technical Field
The present inventions relates to a humidifier for a fuel cell system using moist air discharged from a cathode of a fuel cell stack. More particularly, the present invention relates to a humidifier for a fuel cell system capable of more efficiently humidifying tube side air supplied to a fuel cell stack to be supplied in a high humidity state.
(b) Background Art
As an example of a fuel cell, a polymer electrolyte membrane fuel cell (PEMFC) which has been mainly researched as a power supply source for driving a vehicle is configured to include a membrane electrode assembly (MEA) in which electrode layers, in which an electrochemical reaction between hydrogen and oxygen is caused, are attached to both sides of an electrolyte membrane to which proton moves, a gas diffusion layer (GDL) serving to uniformly distribute reaction gases and transfer generated electric energy, and a bipolar plate that moves the reaction gases and cooling water.
In the fuel cell, hydrogen (e.g., fuel) and oxygen (e.g., air, an oxidizing agent) are each supplied to an anode and a cathode, which are the electrode layers of the membrane electrode assembly, through a channel of the bipolar plate, in which the hydrogen is supplied to the anode and the oxygen (air) is supplied to the cathode. The hydrogen supplied to the anode is decomposed into proton (H+) and electron (e−) by a catalyst of the electrode layer. Further, only the protons selectively pass through the electrolyte membrane (e.g., a cation exchange membrane) and then are transferred to the cathode, and simultaneously the electrons are transferred to the cathode through the gas diffusion layer and the bipolar plate, which is a conductor.
In the cathode, a reaction to generate water by meeting the proton supplied through the electrolyte membrane and the electron transferred through the bipolar plate with the oxygen in the air supplied to the cathode by an air supply apparatus is caused. In particular, a flow of electrons through an external conducting line is generated due to the movement of the generated proton and a current is generated due to the flow of electrons.
Meanwhile, for an operation of the polymer electrolyte membrane fuel cell, moisture is positively necessary. Accordingly, the air supplied to the fuel cell needs to be humidified by a humidifier. A bubbler type, an injection type, an adsorbent type, and the like are various types of humidifiers that are used. However, since a fuel cell vehicle has a restriction in a package size, a membrane humidifier having a relatively reduced volume has been applied. The membrane humidifier has an advantage in respect to the package size and that special electric power is not required.
FIG. 1 is an exemplary diagram schematically illustrating a state in which air is humidified by a general humidifier (membrane humidifier) for a fuel cell. As illustrated in FIG. 1, external tube side air is forcibly blown by an air blower 1 and passes through the membrane humidifier 10. In particular, supersaturation moist air including water discharged from an outlet of a fuel cell stack 20 passes through the membrane humidifier 10 to provide a moisture exchange between the supersaturation moist air and the tube side air to humidify the tube side air, in which the humidified air is supplied to the fuel cell stack 20.
In particular, water is generated from the cathode of the stack 20 and is discharged in a steam (or droplet) state along with non-reaction air and the membrane humidifier 10 is configured to perform moisture exchange and heat exchange to supply air having substantially high humidity relative to the stack 20. The humidity of the air supplied to the stack 20 is a sensitive operation variable which determines an output and durability of the fuel cell. As illustrated in FIG. 1, to improve an operation characteristic, a bypass line 2 configured to bypass the membrane humidifier 10 and a valve 3 are applied based on operation conditions.
A general membrane humidifier which is a gas to gas membrane humidifier using hollow fibers may implement high integration of the hollow fibers having a wide contact surface area to sufficiently humidify the fuel cell stack within a minimal capacity and recovers and reuses moisture and heat included in the gas which is discharged at an increased temperature from the fuel cell stack through the membrane humidifier, thereby separately saving moisture and energy required for the humidification of the fuel cell stack.
A moisture content may be maintained by supplying moisture of a predetermined amount or greater to an ionomer within the electrolyte membrane and a catalyst layer of the membrane electrode assembly which are core components of the fuel cell, to improve performance of ion conductivity which is performed by the electrolyte membrane and the ionomer. Herein, the membrane humidifier is configured to supply the moisture and the heat included in the gas discharged at the increased temperature from the fuel cell stack to dry reacting gas at a normal temperature (e.g., a temperature that is not increased) supplied to the fuel cell stack through a surface of the membrane to humidify the fuel cell stack and maintain a temperature of the fuel cell stack.
Hereinafter, a structure of the general membrane humidifier will be described in detail. In the accompanying drawings, FIG. 2 is an exemplary transversal cross-sectional view schematically illustrating a general membrane humidifier and FIG. 3 is an exemplary cross-sectional view taken along A-A of FIG. 2. As illustrated in FIGS. 2 and 3, the general membrane humidifier 10 includes a manifold including air inlets 11 and 12 and air outlets 13 and 14 and hollow fibers 16 fixed inside the manifold 15, in which the hollow fibers 16 may be attached and fixed to the manifold 16 by plastic such as polyurethane.
In particular, the hollow fiber 16 is provided in plural and is densely embedded in a bundle form. Both ends of a bundle 17 of hollow fibers are fixedly bonded to a potting part 18 disposed inside the manifold 15 and thus the bundle 17 of hollow fibers is fixed inside the manifold 15. The membrane humidifier 10 is configured to perform heat and water exchange between tube side air and over-humidification air in the hollow fibers 16. In particular, uniformity of a gas flow increases humidification efficiency of the humidifier. However, the tube side air has a substantially high uniform flow characteristic, but uniformity of a flow may not be maintained in a shell side space of a section of the humidifier in which the over-humidification air supplying moisture flows since an empty space is formed in an outer region of the manifold due to a lack of the hollow fiber.
In other words, the high-temperature moist air (e.g., over-humidification air) flows along a path having the least resistance and a substantially central portion of the humidifier 10 is densely provided with the hollow fibers 16, and thus it may be difficult for the over-humidification air containing a substantial amount of moisture to reach the substantially central portion of the humidifier 10 and most of the over-humidification air flows in a space of the shell side of the section of the humidifier. Therefore, the hollow fiber disposed in the space of the shell side of the humidifier 10 has low contribution to humidification and the over-humidification air flowing in an exterior of the humidifier 10 has the reduced temperature due to a heat transfer to the exterior and is condensed into water, such that a humidification effect may be further reduced.
To improve the problem of the existing membrane humidifier, a membrane humidifier for a fuel cell system to which a channel opening and closing valve is applied has been developed. In the accompanying drawings, FIG. 4 is an exemplary configuration diagram schematically illustrating the existing membrane humidifier for a fuel cell system to which the channel opening and closing valve is applied and FIG. 5 is an exemplary diagram schematically illustrating an operating state depending on an output section of the membrane humidifier for a fuel cell system.
As illustrated in FIG. 4, the existing membrane humidifier 10 for a fuel cell includes a channel opening and closing valve 19 at a back end of the bundle 17 of hollow fibers adjacent to the humidification air outlet 14 through which the humidification air supplied to the fuel cell stack is discharged. In particular, the channel opening and closing valve 19 is integrally disposed at a distal end of a tube side air supply tube 21 which partitions an exterior and a substantially central portion of the bundle of hollow fibers and both ends of the tube side air supply tube 21 are fixedly supported by the potting part 18 inside the manifold 15. In other words, the channel opening and closing valve 19 is fixedly mounted on the potting part 18 within the manifold 15.
As described above, For the general membrane humidifier, since the tube side air mainly flows in the central portion of the bundle of hollow fibers and the moist air (or over-humidification air) mainly flows in the exterior of the bundle of hollow fibers, and as illustrated in FIG. 4, the channel opening and closing valve 19 is applied to the central portion of the bundle 17 of hollow fibers inside the membrane humidifier 10 to solve a distribution problem of the tube side air and the moist air. In particular, as the channel opening and closing valve 19, a valve which is operated to be opened at a predetermined pressure or greater is used.
Therefore, the valve is closed in a low flow rate condition (e.g., low output condition) to cause the tube side air supplied from the air blower 1 to flow in the outer region of the bundle 17 of hollow fibers, to mainly humidify the exterior of the membrane humidifier 10 and as a flow rate (output) is increased, a pressure is increased and thus the valve 19 is slowly opened to cause the tube side air to flow throughout the bundle 17 of hollow fibers, to humidify the whole portion of the membrane humidifier. Consequently, the channel opening and closing valve 19 is applied to increase humidification performance (e.g., efficiency) of the membrane humidifier.
However, in the existing membrane humidifier 10 adopting the channel opening and closing valve 19, it may be difficult to mount the valve on the potting part 18 to which the bundle 17 of hollow fibers is fixed, a structure of the membrane humidifier 10 is complex, and it may be difficult to determine or control a design for the valve opening and closing.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.